%%{ init: { 'flowchart': { 'curve': 'natural' } } }%%
graph LR
START[ ] -- x1 --> f[y = 3 + 0.5*x1 + 2*x]
START2[ ] -- x2 --> f
f -- y --> STOP[ ]
style START fill-opacity:0, stroke-opacity:0;
style START2 fill-opacity:0, stroke-opacity:0;
style STOP fill-opacity:0, stroke-opacity:0;